TESLA GRADIENT RECALLED ECHO CHARACTERISTICS OF PHOTIC STIMULATION-INDUCED SIGNAL CHANGES IN THE HUMAN PRIMARY VISUAL-CORTEX

Multi-echo measurements of photic stimulation-induced signal changes i n human visual cortex were made at 4 Tesla in order to quantity the na ture of the signal change and its vascular origin, and to determine th e optimum echo time for detection of the changes. Utilizing high resol ution images, two distinct regions (ascribed to be microvasculature an d visible venous vessels) were identified as giving rise to the signal increase. The fractional signal changes in gray matter areas depended linearly on echo time (TE) in the range of 10 to 60 ms and extrapolat ed to virtually zero for TE = 0, indicating that in-flow effects secon dary to stimulation-induced blood flow increases were negligible in ou r functional imaging studies; instead, signal change due to photic sti mulation originated from the increase in the apparent transverse relax ation rate, 1/T2. This decrease in (1/T2*), brought about by the alte rations in hemodynamic parameters, was 1.3 +/- 0.4 s-1 for gray matter and 3.0 +/- 0.7 s-1 (averaged over 10 individuals) for venous vessels visible in the images. The optimum choice of echo time was found to b e TE greater-than-or-equal-to T2.